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Abstract:

A planar light-emitting device includes a light source element and a
light guide plate. The light source element is configured to emit light.
The light guide plate has a housing hole, a light-emitting face and a
light reflecting face. The housing hole houses the light source element.
The light reflecting face is formed along a side end portion of the light
guide plate. The light reflecting face has an inclined portion that is
located next to the housing hole in a first direction of the light guide
plate and a flat portion that is located next to the inclined portion in
a second direction that is perpendicular to the first direction. The
inclined portion has an inclination angle with respect to the flat
portion such that the light reflected on the inclined portion is
prevented from returning the housing hole.

Claims:

1. A planar light-emitting device comprising: a light source element
configured to emit light; and a light guide plate having a housing hole
that houses the light source element, a light-emitting face that is
configured to emit the light from the light source element, and a light
reflecting face that is configured to reflect the light from the light
source element, the light reflecting face being formed along a side end
portion of the light guide plate, the light reflecting face having an
inclined portion that is located next to the housing hole in a first
direction of the light guide plate and a flat portion that is located
next to the inclined portion in a second direction that is perpendicular
to the first direction, the inclined portion having an inclination angle
with respect to the flat portion such that the light reflected on the
inclined portion is prevented from returning the housing hole.

2. The planar light-emitting device according to claim 1, wherein the
housing hole of the light guide plate has a circular shape.

3. The planar light-emitting device according to claim 1, wherein the
inclined portion of the light reflecting face has a peaked shape with an
apex portion as viewed from a third direction that is perpendicular to
the first and second directions of the light guide plate, the apex
portion being located closest to the housing hole in the inclined portion
of the light reflecting face, the apex portion being further located at a
center of the inclined portion along the second direction of the light
guide plate.

4. The planar light-emitting device according to claim 3, wherein the
inclined portion of the light reflecting face includes a pair of inclined
faces that is inclined with respect to the flat portion of the light
reflecting face, the inclined faces sandwiching the apex portion in the
second direction of the light guide plate, the inclined faces being
symmetrically formed each other with respect to an imaginary plane of
symmetry that is perpendicular to the second direction of the light guide
plate.

5. The planar light-emitting device according to claim 1, wherein the
inclined portion of the light reflecting face has a width in the second
direction of the light guide plate that is at least equal to a diameter
of the housing hole.

6. The planar light-emitting device according to claim 1, further
comprising a reflective sheet configured to reflect the light from the
light source element toward the light-emitting face, the reflective sheet
being disposed on a bottom face of the light guide plate, the bottom face
of the light guide plate being opposite the light-emitting face in a
third direction that is perpendicular to the first and second directions
of the light guide plate.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to Japanese Patent Application No.
2010-106465 filed on May 6, 2010. The entire disclosure of Japanese
Patent Application No. 2010-106465 is hereby incorporated herein by
reference.

BACKGROUND

[0002] 1. Field of the Invention

[0003] The present invention generally relates to a planar light-emitting
device. More specifically, the present invention relates to a planar
light-emitting device in which a point light-emitting element such as an
LED (Light Emitting Diode) is used as a light source element.

[0006] With the conventional planar light-emitting device in FIG. 5, an
LED 130 is housed as a point light-emitting element in the center of a
circular hole 120 formed in a light guide plate 110. The hole 120 is
located near the flat, side end face of the light guide plate 110. The
above-mentioned side end face is formed as a reflecting face 111 by
applying a reflective sheet, reflective tape, or another such optically
reflective film 140 to this side end face. In FIG. 5, 150 is an LED
substrate on which the LED 130 is mounted.

[0007] With the conventional planar light-emitting device, light that
comes out of the LED 130 moves out through the area around the LED 130,
passes through the walls of the hole 120, and goes into the light guide
plate 110. Light that has entered the light guide plate 110 moves out in
all directions around the hole 120 as indicated by the arrows in FIG. 5,
and comes out of a light emitting face formed by one face of the light
guide plate 110, so that the light guide plate 110 emits planar light.
Also, part of the light that has come out of the LED 130, passed through
the walls of the hole 120, and entered the light guide plate 110 becomes
return light upon being reflected by the reflecting face 111, which is
formed by the flat, side end face of the light guide plate 110.

[0008] Meanwhile, another conventional backlight is also known (see
Japanese Laid-Open Patent Application Publication No. 2006-19141, for
example). This backlight is such that a side face emitting type of LED
disposed on an LED array substrate is disposed in a recess of a light
guide plate. A diffusive and reflective sheet is disposed on the lower
face of the light guide plate, a diffusive sheet on the upper face of the
light guide plate, and a reflective sheet on the side end face of the
light guide plate.

[0009] Furthermore, another conventional backlight is also known (see
Japanese Laid-Open Patent Application Publication No. 2006-351522, for
example). With this backlight, two light guide plates are disposed on
both sides of an LED. Reflective sheets are disposed on the lower faces
of the light guide plates, and an isolation sheet is disposed on the
upper faces of the light guide plates. A peaked reflector is disposed on
the isolation sheet. A diffusing sheet is disposed above and a specific
distance away from the isolation sheet, and an optical sheet is disposed
over this diffusing sheet, forming a hollow portion between the diffusing
sheet and the isolation sheet. A semi-cylindrical reflector is provided
to the side of the light guide plates, and light that comes out of the
side end faces of the light guide plates is guided by the reflector to
the hollow portion, reflected by the surface of the peaked reflector, and
is emitted through the diffusing sheet and the hollow portion to the
outside.

[0010] As yet another known example of prior art, there has been research
into a backlight device in which light utilization efficiency is improved
by disposing a highly reflective member (one with high optical
reflectivity) near an LED light source, so that the brightness of the
liquid crystal panel is improved (see Japanese Laid-Open Patent
Application Publication No. 2004-31064, for example).

SUMMARY

[0011] It has been discovered that with the conventional planar
light-emitting device described through reference to FIG. 5, light that
has become return light upon being reflected by the reflecting face 111
formed by the flat, side end face of the light guide plate 110 (in FIG.
5, the return light is indicated by the arrow labeled A, in particular)
goes through the walls of the hole 120 in the light guide plate 110, is
incident on and absorbed by the hole 120. Because this happens, it is
possible that the return light reflected by the reflecting face 111 will
not be efficiently used to achieve planar light-emission at the light
emitting face of the light guide plate 110.

[0012] The examination results of the emission characteristics of the
planar light-emitting device having this constitution are shown in FIG.
6. Specifically, at the rear part of the light guide plate 110 near where
the LED 130 is installed, or in other words, at the portion of the light
guide plate 110 on the opposite side from the reflecting face 111 with
the hole 120 sandwiched in between, light is focused on both sides of a
region Z1 at the rear part where the LED 130 is installed, and high
brightness regions Z2 appear. But on the other hand, the brightness is
far lower than in the high brightness regions Z2 in the above-mentioned
region Z1 in between these high brightness regions Z2. Furthermore, the
greater the distance from the high brightness regions Z2 in the in-plane
direction of the flat reflecting face 111, there less amount of light is
emitted and the greater is the difference in the quantity of light in
bright/dark regions Z3. Furthermore, it has been discovered that there is
a relatively large difference in brightness at the boundaries between the
high brightness regions Z2 and the bright/dark regions Z3. Also, it has
been discovered that the above-mentioned high brightness regions Z2 are
produced by return light that has been reflected by the reflecting face
111 and passed near the hole 120 (examples are indicated by the arrows
A1).

[0013] It can not be understood from the technology proposed in the
above-mentioned patent application Publications how the high brightness
regions Z2, or the region Z1, the bright/dark regions Z3, and so forth
with far lower brightness than the high brightness regions Z2, appear,
how a relatively large difference in brightness appears at the boundaries
between the high brightness regions Z2 and the bright/dark regions Z3,
and so on.

[0014] The present invention was conceived in light of the above-mentioned
discoveries. One object of the present invention is to provide a planar
light-emitting device with which emission characteristics can be
improved.

[0015] In accordance with one aspect, a planar light-emitting device
includes a light source element and a light guide plate. The light source
element is configured to emit light. The light guide plate has a housing
hole, a light-emitting face and a light reflecting face. The housing hole
houses the light source element. The light-emitting face is configured to
emit the light from the light source element. The light reflecting face
is configured to reflect the light from the light source element. The
light reflecting face is formed along a side end portion of the light
guide plate. The light reflecting face has an inclined portion that is
located next to the housing hole in a first direction of the light guide
plate and a flat portion that is located next to the inclined portion in
a second direction that is perpendicular to the first direction. The
inclined portion has an inclination angle with respect to the flat
portion such that the light reflected on the inclined portion is
prevented from returning the housing hole.

[0016] With the planar light-emitting device, it is possible to provide a
planar light-emitting device with which emission characteristics can be
improved.

[0017] These and other objects, features, aspects and advantages will
become apparent to those skilled in the art from the following detailed
description, which, taken in conjunction with the annexed drawings,
discloses a preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Referring now to the attached drawings which form a part of this
original disclosure:

[0019] FIG. 1 is a top plan view of a planar light-emitting device in
accordance with one embodiment;

[0020] FIG. 2 is a cross sectional view of the planar light-emitting
device taken along II-II line in FIG. 1;

[0021]FIG. 3 is a detailed top plan view of the planar light-emitting
device illustrated in FIG. 1;

[0022] FIG. 4 is a top plan view of the planar light-emitting device
illustrating emission characteristics of the planar light-emitting device
illustrated in FIG. 1;

[0023] FIG. 5 is a detailed top plan view of a conventional planar
light-emitting device; and

[0024]FIG. 6 is a top plan view of the conventional planar light-emitting
device illustrating in FIG. 5.

DETAILED DESCRIPTION OF EMBODIMENTS

[0025] A preferred embodiment will now be explained with reference to the
drawings. It will be apparent to those skilled in the art from these
disclosures that the following descriptions of the preferred embodiment
are provided for illustration only and not for the purpose of limiting
the invention as defined by the appended claims and their equivalents.

[0026] As shown in FIGS. 1-3, a planar light-emitting device in this
embodiment includes a light guide plate 10 with a circular housing hole
20, a reflective sheet 12, a reflector 21, an LED (Light-Emitting Diode)
(e.g., light source element) 30, an optically reflective film 40 and a
substrate 50. The light guide plate 10 is made of a resin, such as an
acrylic resin and the like. The light guide plate 10 is integrally formed
as a one-piece, unitary member. The planar light-emitting device is used
as a backlight for a liquid crystal panel, a liquid crystal module, a
liquid crystal display device or the like. The LED 30 serves as a point
light-emitting element. The LED 30 is mounted on the substrate 50. The
LED 30 is housed in a center portion of the hole 20. The hole 20 is
formed in the light guide plate 10. The reflective sheet 12 has a
circular cut-out where it overlaps the hole 20. The reflective sheet 12
is provided on a lower face (e.g., bottom face) of the light guide plate
10. The lower face of the light guide plate 10 is formed as a
non-emitting face. On the other hand, an upper face of the light guide
plate 10 is formed as a light emitting face 13 that allows light to pass
through and through which the light emits. Therefore, the non-emitting
face is formed by the lower face on the opposite side from the light
emitting face 13. The reflective sheet 12 reflects light from the LED 30
toward the light emitting face 13. The LED 30 is installed on the
non-emitting face side of the light guide plate 10. The LED 30 is housed
in the hole 20. The LED 30 is disposed through the circular cut-out part
of the reflective sheet 12.

[0027] The light guide plate 10 includes a side end face having a flat
side end face 10a and a pair of inclined faces 10b. The hole 20 is
located near the side face of the light guide plate 10. The reflector 21
is installed at an upper part inside the hole 20. The reflector 21
reflects light that comes out of the LED 30 toward radial directions of
the LED 30. The reflector 21 is separately formed as an independent
member of the light guide plate 10. However, the reflector 21 can be
integrally formed with the light guide plate 10 as a one-piece, unitary
member. The side end face of the light guide plate 10 is formed as a
reflecting face 11 by applying a reflective sheet, reflective tape, or
another such optically reflective film 40 to this side end face. The LED
30 is mounted on the LED substrate 50.

[0028] As shown in FIG. 3, in this embodiment, the reflecting face 11 that
is formed by the side end face of the light guide plate 10 is segmented
into a hole-facing region (e.g., inclined portion) A1 that includes a
portion located nearest the hole 20, and a region that excludes this
hole-facing region A1, or in other words, a flat regions (e.g., flat
portion) A2 on both sides of the hole-facing region A1. In the
hole-facing region A1, the reflecting face 11 changes shape to a recess
that is V-shaped (e.g., peaked shape) in top plan view (e.g., when viewed
in a third direction). The flat regions A2 on both sides of the
hole-facing region A1 remain in the flat shape.

[0029] In this embodiment, the width of the hole-facing region A1 in a
widthwise direction (e.g., second direction) of the flat side end face
10a along an upper edge of the flat side end face 10a is set to be about
the same as the diameter D of the hole 20 in the light guide plate 10.
Specifically, the width of the hole-facing region A1 is at least equal to
the radius of the hole 20. Preferably, the width of the hole-facing
region A1 is at least equal to the diameter D of the hole 20. In the
hole-facing region A1, the recessed reflecting face 11 is formed in a
peaked shape whose apex is located at the center of the hole-facing
region A1 in the widthwise direction. This apex (i.e., widthwise center
of the hole-facing region A1) a is located at a location closest to the
hole 20 in a lateral direction that perpendicular to the widthwise
direction. Two side portions that flank this apex a in the widthwise
direction are formed as the inclined faces 10b having a symmetrical, flat
surface. As shown in FIG. 3, the hole-facing region A1 is located next to
the hole 20 in the lateral direction. The flat regions A2 are located
next to the hole-facing region A1 in the widthwise direction. The
inclined faces 10b are symmetrically formed each other with respect to an
imaginary plane I of symmetry that is perpendicular to the widthwise
direction.

[0030] With the planar light-emitting device, the light emitted by the LED
30 move out in the radial directions around the LED 30, pass through an
inner wall of the hole 20, and go into the light guide plate 10. The
light that enters the light guide plate 10 moves out in all directions
around the hole 20 as indicated by the arrows L2 in FIG. 3, and comes out
of the light emitting face 13 formed by the upper face of the light guide
plate 10. As a result, the light guide plate 10 emits planar light. Also,
part of the light that comes out of the LED 30, passes through the inner
wall of the hole 20, and enters the light guide plate 10 becomes return
light upon being reflected by the entire reflecting face 11, which is
formed by the side end face of the light guide plate 10.

[0031] Of this return light, the light reflected by the reflecting face 11
formed by the inclined faces 10b moves in a direction corresponding to
the inclination angles of the inclined faces 10b (the inclination angles
of the inclined faces 10b with respect to the flat side end face 10a of
the reflecting face 11 of the flat region A2). In this embodiment, the
inclination angles of the inclined faces 10b is set so that the return
light reflected by the reflecting face 11 formed by the inclined faces
10b included in the hole-facing region A1 moves in a direction that does
not return into the hole 20, as indicated by the arrows L1 in FIG. 3. In
other words, the inclination angles of the inclined faces 10b is set such
that the light that directory incidents from the LED 30 and reflects on
the inclined faces 10b is prevented from returning or entering the hole.
Accordingly, either the amount of return light L1 reflected by the
reflecting face 11 formed by the inclined faces 10b that goes into the
hole 20 is reduced, or the return light L1 does not go into the hole 20
at all. Rather the return light reflected by the inclined faces 10b moves
to the sides of the hole 20 and emits from the light emitting face 13.
Thus, this return light L1 is utilized more effectively for planar light
emission.

[0032] The emission characteristics of the planar light-emitting device
pertaining to this embodiment are examined. The results are schematically
shown in FIG. 4. Specifically, the return light reflected by the
hole-facing region A1 and the flat regions A2 of the reflecting face 11
is less likely to go into the hole 20 and be absorbed. In particular, the
return light L1 reflected by the reflecting face 11 included in the
hole-facing region A1 and formed by the inclined faces 10b splits up and
moves to the two sides flanking the hole 20. Thus, this return light L1
mixes with the return light reflected by the reflecting face 11 included
in the flat regions A2, making it harder to distinguish the region Z1,
the high brightness regions Z2, and the bright/dark regions Z3 described
through reference to FIG. 6. More specifically, with the light emitting
face 13 of the light guide plate 10 shown in FIG. 4, the greatest amount
of light is at a back portion P of the hole 20 that extends in the
lateral direction from the hole 20. Then, this portion P becomes the
brightest region in the light emitting face 13. Furthermore, brightness
decreases as moving away from this portion P in the widthwise directions
of the arrows Y1 and Y2 toward the sides of the hole 20. However, there
is not that much difference in the quantity of light between the bright
and dark portions (i.e., between the portion P and the portions spaced
apart from the portion P in the lateral directions).

[0033] This will now be described through contrast with the emission
characteristics shown in FIG. 6. Regarding the difference in the quantity
of light (bright/dark) of the light emitting face within given regions
behind the holes 20, 120, with the emission characteristics shown in FIG.
6, there is a large difference in brightness between the region Z1 and
the two high brightness regions Z2. Furthermore, even among the
bright/dark regions Z3, there is a large difference in brightness between
the bright and dark portions. Moreover, there is a large difference in
brightness at the boundaries between the high brightness regions Z2 and
the bright/dark regions Z3. On the other hand, with the emission
characteristics shown in FIG. 4, the difference in brightness within the
corresponding regions of the light emitting face 13 corresponding to the
regions Z1 to Z3 shown in FIG. 6, respectively, is reduced. Specifically,
the region Z1 with far lower brightness and the high brightness regions
Z2 with far higher brightness are less likely to appear, and there will
be less of a difference in brightness at the boundaries between the high
brightness regions Z2 and the bright/dark regions Z3. Furthermore, there
will be less of a difference in the amounts of light between the bright
and dark portions of the bright/dark regions Z3. Therefore, there is less
discrepancy in the quantity of light over the entire light emitting face
13 of the light guide plate 10, and the light is emitted in a uniform
brightness. Thus, the emission characteristics are improved over those of
a conventional planar light-emitting device.

[0034] With this embodiment, the reflecting face 11 included in the
hole-facing region A1 has a peaked shape, and the inclined faces 10b are
formed as flat faces. However, the reflecting face 11 can have some other
shape. For example, it is also possible for the inclined faces 10b to be
formed as curved faces, respectively. In other words, the shape of these
parts should be decided such that the amount of return light that is
reflected by the reflecting face 11 included in the hole-facing region A1
and that goes into the hole 20 is reduced relative to a case in which the
hole-facing region A1 has a flat side face as a conventional planar
light-emitting device.

[0035] With the planar light-emitting device pertaining to this
embodiment, the reflecting face 11 formed by the side end face of the
light guide plate 10 is segmented into the hole-facing region A1 that
includes the portion located the shortest distance from the hole 20 that
houses the LED (e.g., point light-emitting element) 30, and the flat
regions A2 on both sides thereof. The reflecting face 11 in the
hole-facing region A1 is formed in a peaked shape, for example. Thus, the
return light reflected by the reflecting face 11 efficiently reaches the
bright/dark regions Z3 shown in FIG. 6. Accordingly, not only is there
less of a difference in the quantity of light between the bright and dark
portions of the bright/dark regions Z3 shown in FIG. 6, but there is a
reduction in the quantity of light in the high brightness regions Z2
shown in FIG. 6. As a result, there is less of a difference between the
quantity of light in the region Z1 and the quantity of light in the
regions Z2. Furthermore, there is less of a difference in the brightness
at the boundaries between the high brightness regions Z2 and the
bright/dark regions Z3 shown in FIG. 6. Therefore, there is less of a
difference in the quantity of light over the entire light emitting face
13 of the light guide plate 10. Thus, the emitted light is of more
uniform brightness, and the emission characteristics of the planar
light-emitting device are improved.

[0036] Also, this effect is achieved merely by changing the shape of part
of the side end face that forms the reflecting face 11 of the light guide
plate 10, so an advantage is that there is no need to add any special
parts.

[0037] With the planar light-emitting device, the device merely includes
the LED 30 at one location as a light source. However, the device can
include a plurality of LEDs 30 as a light source. In this case, the light
guide plate 10 have a plurality of pairs of inclined faces 10b on the
side end face of the light guide plate 10 at spaced apart locations along
the side end face, and a plurality of holes 20 at locations corresponding
to the pairs of the inclined faces 10b. Then, the LEDs 30 are housed in
the holes 20, respectively.

General Interpretation of Terms

[0038] In understanding the scope of the present invention, the term
"comprising" and its derivatives, as used herein, are intended to be open
ended terms that specify the presence of the stated features, elements,
components and groups, but do not exclude the presence of other unstated
features, elements, components and groups. The foregoing also applies to
words having similar meanings such as the terms, "including", "having"
and their derivatives. Also, the terms "part," "section," "portion,"
"member" or "element" when used in the singular can have the dual meaning
of a single part or a plurality of parts. As used herein to describe the
present invention, the following directional teems "forward, rearward,
above, downward, vertical, horizontal, below and transverse" as well as
any other similar directional terms refer to those directions of a planar
light-emitting device equipped with the present invention. Accordingly,
these terms, as utilized to describe the present invention should be
interpreted relative to a planar light-emitting device equipped with the
present invention as used in the normal operating position.

[0039] While a preferred embodiment have been chosen to illustrate the
present invention, it will be apparent to those skilled in the art from
these disclosures that various changes and modifications can be made
herein without departing from the scope of the invention as defined in
the appended claims. Furthermore, the foregoing descriptions of the
preferred embodiment according to the present invention are provided for
illustration only, and not for the purpose of limiting the invention as
defined by the appended claims and their equivalents.